The present invention relates to the broad field of FM waveform generation and more particularly to the field of linear frequency modulation. In even greater particularity, the present invention relates to linear frequency modulation having a large time-bandwidth product and an extremely linear frequency versus time waveform.
Previous known methods of linear frequency modulation could generate extremely linear frequency versus time waveforms for very short times only, or they could generate long frequency versus time waveforms with relatively poor linearity. Utilized in some of the prior art was a 1.0 GHz clock-rate scheme in which a closed loop system would be clocked at a 1.0 GHz rate; however, inherent in such a scheme are problems with reliable clocking of digital devices at the 1.0 GHz rate.
Another scheme for achieving the desired time-bandwidth product was digital computation with a single sideband generator; however, this method required a 10ns digital to analog converter and sample and hold circuits. Also, the scheme required phase matching over an 8 megahertz range within 1.degree. in the filters and mixers of the single sideband generator.
Another method is the use of digitally controlled X-band phase shifters; however, this method encounters problems in the precise timing in the phase shifters and the number of bits necessary to drive the phase shifters.
The closest prior art would be the use of analog frequency modulators using an analog feedback circuit, wherein the major problem is the lock-up time required for the loop between LFM strokes which yields a lower duty factor than desired in utilization.